Conventionally, as an air conditioner outdoor unit, for example, there is a structure in which a blowing fan is arranged in an upper portion of a casing and a heat exchanger having a U-shaped cross section is arranged within the casing.
In the case that the air conditioner outdoor unit having the structure mentioned above is made compact, a space between the heat exchanger and a top surface of the casing is reduced in size. Accordingly, the distance between the blowing fan, and the heat exchanger and the top surface of the casing is unavoidably shortened. In this case, if a structural object exists upstream of the blowing fan, suction air flow of the fan may be obstructed. Accordingly, as a countermeasure thereof, the fan is arranged in a state in which the center of the fan and the structural object are eccentric.
Structural objects existing upstream of the blowing fan include a fan motor and a motor stator supporting the fan motor. In the air conditioner outdoor unit, the fan motor is generally arranged in a suction side of the blowing fan, and the height direction of the outdoor unit is made compact. If a blowing fan having a large diameter (in this case, the height in an axial direction becomes larger in addition to an outer diameter) is arranged as the blower of the outdoor unit mentioned above, an arranging space of the fan motor and the motor stator is reduced in size, and a turbulence of a suction air flow is generated by the motor stay, so that an NZ noise (a peak noise having a frequency equal to the product NZ of the number of impeller blades N and a rotation speed Z of the blowing fan) may become loud. Accordingly, there has been proposed a structure intending to solve the problem mentioned above by curving or bending the motor stay in a direction away from the blowing fan (refer to Patent Document 1).
Further, in the case that the air conditioner outdoor unit is made compact, the blowing fan and the casing are arranged close to each other. As a result, there is a problem that a side surface of the casing and an impeller of the blowing fan are interfered with each other, and the NZ noise is increased. Particularly, in the case that the heat exchanger is not provided on a whole side surface of the casing, the suction air flow drifts in the rotating direction of the blowing fan. Accordingly, there is a problem that the NZ noise tends to become loud.
As a method of suppressing the increase of the NZ noise mentioned above, there has been proposed a structure which is designed to suppress the increase of the NZ peak noise by employing a variable pitch impeller in which an interval of an attaching pitch of the impeller blades in the rotating direction of the impellers is uneven, thereby slightly shifting an interference cycle between the impeller blade and the side surface of the casing (refer to Patent Document 2).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-130394    Patent Document 2: Japanese Laid-Open Patent Publication No. 5-223093